1. Field of the Invention
This invention relates to circuits for maintaining constant voltage on utility distribution networks and, more particularly, circuits for dynamically compensating for transient electrical disturbances in the supply to a distribution network so as to reduce and substantially eliminate transient interruptions such as voltage sags.
2. Description of the Prior Art
In recent years, quality requirements related to the delivery of electric power have increased dramatically. Modern automated manufacturing and process controls use complex machinery and data handling equipment which employ massive amounts of sophisticated circuits, e.g., solid-state electronic switching circuits and the like. Such circuits rely on substantially disturbance-free electric power which is provided by a utility distribution network. Due to their basic operation and internal structure, many modern electric circuits are sensitive to electrical disturbances and noises which may cause them to malfunction or even to fail, in turn interrupting or shutting down completely manufacturing and other processes. For example, it is well known that digital computers, and thus computer-controlled system loads, are critically sensitive to maintenance of a uniform utility signal. Consequently, interruptions or disturbances on the side of the distribution network which supplies power to these loads can cause significant waste in production time and material and manufacturing, and substantial monetary losses in commercial operations.
Disturbances in electrical utility supply networks (transmission and distribution) might be caused for a variety of reasons. Power equipment faults and insulation failures, line switchings, capacitor switchings, large transformer and motor energizations, and non-linear loads such as arc furnaces, variable speed drives, rectifiers, etc. are only a few such causes. Such transient disturbances, as distinguished from complete power failure, manifest themselves as momentary power supply interruptions, voltage sags, voltage transients, voltage magnitude variations, and harmonics. Thus, voltage sags and transients may be caused by failures or switchings within a particular transmission or distribution line. Moreover, several lines in the transmission and distribution system may be tied to a single bus, such that these lines in parallel connection in effect feed a common bus. Because of this nature of power transmission and distribution networks, single line faults and switchings frequently do not result in power interruptions at the bus or in another line connected to that bus, but only in voltage sags and transients. The magnitude of such sags and transients is dependent upon the length and impedance characteristic of the lines involved in the overall network. Likewise, harmonic voltage components may be produced across the line impedance by harmonic load currents drawn by non-linear loads. These and other like disturbances are transient in nature, and are usually manifested by a change in peak-to-peak amplitude of the utility voltage and/or transient spikes and harmonics. Hereinafter, the term "disturbances" will refer to any of these conditions.
The field of this invention is that of providing a dynamic response to a power distribution line which carries power to a number of loads, at least some of which are sensitive to power line disturbances of the type described above. The field is similar to, but distinct from, that of the uninterruptible power supply (UPS) which is designed to interface between the utility-provided electric power line and critical equipment, such as a computer or data storage element. The UPS concept is to have a DC energy device, usually a battery, which is maintained in a charged state by connection to the power line, and to employ a dc to ac inverter to convert the dc battery power to the desired ac power. This is accomplished by generating a single-phase (or three-phase) alternating voltage which is identical to the normal utility voltage, i.e., has the same frequency and amplitude, and is synchronism with it. There are two basic philosophies with regard to UPS devices. In one basic arrangement, the battery/inverter system normally provides the power for the critical load and the power line is used to keep the battery charged. A bypass switch is employed to get the utility power direct to the load only if the power/inverter system fails. This arrangement provides a substantially total isolation from the utility power system, and thus can provide a small amount of power that is essentially free of system disturbances. However, it has the disadvantage of requiring power conversion equipment that is operated normally at rated power and, accordingly, may introduce appreciable losses, particularly at higher power levels. Such an approach is not feasible for dynamic compensation of transient disturbances on a distribution line that is providing a significant amount of power to the loads attached to it.
In another UPS arrangement, the battery/inverter is connected essentially in parallel with the utility power system, but maintained normally in a stand-by state so that the power demand of the load is provided by the utility system via a fast circuit breaker (preferably solid-state) which can isolate the faulty utility system from the load and the battery/inverter power supply. With such a UPS, in the case of a power system disturbance or loss of power for a significant time, the circuit breaker is opened and the load is supplied directly from the battery/inverter system. When the power system is restored to its normal operation, the breaker is closed and the battery/inverter system goes back into stand-by, with power being supplied directly from the utility line. In this arrangement, the power conversion equipment is again fully rated, but the operating losses and cost are lower. The disadvantage of this arrangement is that it is difficult to make it responsive to fast dynamic transient conditions, or disturbances of relatively short duration, such that transients and harmonics can get to the load. Also, if a disturbance is recognized and reacted to quickly, then the battery must supply the entire load for the duration of the disturbance. While this is an adequate solution for relatively small power-consuming loads, it is not an appropriate solution for a utility distribution network which at any given moment may be providing a significant load. Consequently, the UPS philosophy is not acceptable to the utility environment which this invention addresses.
For voltage regulation of a utility line, the method of series (i.e., capacitive) line impedance compensation is known in the art. Thus, it is known that voltage variations in transmission and distribution systems may be caused by the voltage drop developing across the normally inductive series line impedance as a result of changing line (load) current. This variation in voltage available on the line can be reduced by partially cancelling the line inductance, which is done by connecting a predetermined amount of capacitance in series with the line. The function of the series capacitor is effectively to inject a voltage into the line at the fundamental frequency so as to oppose the voltage drop developed across the inductive line impedance at the same frequency. This results in reducing the voltage drop to an equivalent to that of a shorter utility line with a smaller inductance. Further, it is known that if an ac voltage which has a quadrature phase relationship with the line current at the fundamental frequency (i.e., the voltage lags the current by 90 electrical degrees), the amplitude of which is made portional to that of the line current, is injected in series with the line, a series compensation functionally equivalent to that produced by a series capacitor is obtained. This technique, utilizing a solid-state inverter for injecting the compensating signal in series with the line, is disclosed in U.S. Pat. No. 5,198,746, issued Mar. 30, 1993, and assigned to the same assignee as this invention. With this technique, the magnitude of the inverter output voltage which is inserted in series with the line can be varied continuously, and its polarity can be changed from that representative of a capacitor to that of an inductor, whereby the effective line impedance can be varied over a wide range. This technique is very efficient, and permits steady state maintenance of the line voltage at a substantially constant amplitude even in the face of large line current variations. It does not, however, meet the need for dynamic compensation of disturbances which require a real power input to the line in order to effectuate compensation. Thus, in the case of voltage sags, or transients which represent a dynamic change in real power which would otherwise be delivered to the load, there remains a need for an efficient and reliable mechanism for responding so as to minimize variations in the distribution line signal delivered to the loads that are tied to it.